gui2/taddon_description: Use a [stacked_widget] so URL controls may be hidden

The URL controls overlay a placeholder "None" label using the
[stacked_widget]. When there's no URL to show, the URL controls are
hidden. When there is a URL to show, the placeholder is hidden instead.
gui2/teditor_edit_scenario: Change XP modifier slider step and minimum values
gui2/teditor_edit_scenario: Unify some grids for a more orderly appearance